1. Technical Field
Aspects of this document relate generally to telecommunication systems and techniques for transmitting data across a telecommunication channel.
2. Background Art
Satellite Carrier Cancellation links can be adversely affected by modulation schemes that cause signal correlation both between a given signal and a delayed version of itself, or periodically between the two signals that need to separated using cancellation techniques.
The conventional Carrier Adaptive Cancellation technique assumes that the desired receive (Rx) signal and the interfering transmit (Tx) signal to be cancelled are different enough that complete cancellation of the delayed transmit signal out of the composite signal will optimally extract the desired Rx signal. This is true for many types of satellite carriers, but problems arise in some cases. A case in point is standardized modulations such as, but not limited to, DVB-S2. DVB-S2 is a specification for Digital Video Broadcasting, defined by the European Telecommunications Standards Institute (ETSI). This specification is identified as ETSI EN 302 307, “Digital Video Broadcasting (DVB) Second generation framing structure channel coding and modulation systems for Broadcasting, Interactive Services, News Gathering and other broadband satellite applications” and is herein incorporated.
Performance degrading correlation can arise here due to the following aspects of the standardized system:
1. The Error Correction engine is standardized on a Low Density Parity Check (LDPC) systematic code and uses a synchronous scrambler. Systematic codes transmit the original message to which parity bits are appended. The input data can be largely repetitive, for example, in the case of test signals or null data. Data scrambling is synchronous for this standard, meaning that each data frame (codeword) gets mixed with the same randomizer signal. Correlation can and does occur during the systematic portion of a data frame.
2. The Physical Layer at the start of each data frame (codeword) is not scrambled, and if the same modulation and coding is used, it generates an identical 90 symbol modulation pattern at the start of each frame.
3. The DVB-S2 standard specifies that in the absence of enough input data being sent to fill all of the data frames (codewords) that the system send short null frames, called Physical Layer Dummy Frames (PLDFs). These PLDFs exactly repeat the modulated sequence every 3330 modulated symbols.
Thus, a need exists for a method of reducing or preventing this correlation to allow robust link performance.